A conductivity sensor, which has a circularly cylindrical housing, wherein metal measuring electrodes are arranged planarly on a circular end wall of the cylindrical housing is known from EP 1089072 A2. The metal measuring electrodes form, in such a case, two voltage electrodes and two electrical current electrodes. The voltage electrodes are circularly embodied and surrounded by the two flat electrical current electrodes, which essentially extend in a semicircle. This arrangement has the disadvantage that it lacks rotational symmetry, which is objectionable due to an undefined end stop in the case of an installation in a screw lid, for example. The mounting of the electrode structure in the conductivity sensor is made difficult by such an undefined installation factor.
A measuring cell for measuring and monitoring the electrical conductivity of a liquid is known from U.S. Pat. No. 4,227,151. This measuring cell has a plurality of circularly shaped electrodes, which are mutually arranged at a distance, and electrically insulated, from one another. In such case, at least four concentric circular electrodes are arranged in or on a planar surface and isolated from one another by regions of electrically non-conducting material. The innermost of the four electrodes is hollow, in order to accommodate a temperature sensitive element. Such a hollow embodied electrode leads to sealing problems, since the described conductivity sensor is used for measuring the physical properties of a liquid.
A conductivity measured value transducer is known from EP 0386660 A1. Four concentric metal rings are placed as electrodes for the conductivity measured value transducer on the end face of a cylindrical support body of insulating material. In such case the electrodes are planar with the end face. In such case, the electrodes with a greater cross section are applied as electrical current electrodes, while the electrodes with a smaller cross section are applied as voltage electrodes. A disadvantage in this case is that this electrode structure requires a relatively large space due to the different cross sections of the electrodes.